This invention relates to acoustic well logging apparatus and more particularly to apparatus for attenuating and delaying acoustic energy.
An acoustic well logging apparatus is usually cylindrically shaped and suitably sized for passage through a fluid filled borehole. In the present day form of acoustic well logging apparatus, a transmitter is energized with a pulse of energy to emit acoustic energy into the borehole fluid surrounding the investigating apparatus for passage to the formation surrounding the borehole. The acoustic energy travels through the formation and a portion thereof is detected by one or more nearby acoustic receivers after passing from the formation back into the borehole. In some presently used systems, there are a plurality of acoustic transmitters and receivers.
In the most common type of acoustic logging apparatus, the travel time for acoustic energy to travel between the transmitter and receiver is measured. This is accomplished by generating a pulse which has a known time relationship with the firing of the transmitter and generating a second pulse coincident with the detection of one of the initial half-cycles of the acoustic energy arriving at the receiver. These pulses can then be processed to obtain numerical indications of the travel time between the transmitter and receiver. When making such measurements of the travel time of the formation adjoining a borehole, it is imperative that the acoustic energy travelling through the formation arrive at the receiver before the energy travelling through the borehole. Usually the velocity of acoustic energy is significantly less in a borehole fluid than in the formation. However the acoustic velocity in a longitudinal support member constructed of steel will be much greater than in the formations. Of course it would be possible to construct the longitudinal support member of a low velocity material such as rubber but this would severely decrease the mechanical strength and rigidity of the support member and is undesirable for this reason.
This problem has been solved to a large extent by constructing the support member in a manner to delay the acoustic energy. Such a construction is shown in U.S. Pat No. 3,191,141 granted to N. A. Schuster on June 22, 1965. In this Schuster structure, a support member having a generally tubular configuration has a plurality of void or open spaces arranged about the periphery of the tubular member and along its length so as to provide a tortuous path for acoustic energy attempting to pass longitudinally along the support member. This arrangement is particularly useful for delaying acoustic energy at low frequencies. Other apparatus for providing such a delay for acoustic energy can be found in U.S. Pat. No. 3,381,267 granted to W. E. Cubberly, Jr., et al on Apr. 30, 1960; U.S. Pat. No. 3,191,142 granted to Maurice P. Lebourg on June 22, 1965, and U.S. Pat. No. 3,190,388 granted to E. I. Moser et al on June 22, 1965. As stated in the above mentioned patents, this acoustic delay structure will also provide some attenuation of acoustic energy because of the constantly varying shape and size of the acoustic path. As shown in the above-mentioned Moser et al patent, weights can be added to the periphery of this tubular member to enhance this effect.
Another prior-art technique for preventing passage of acoustic energy along the support member has been to construct the support member as a so-called mechanical filter which operates to attenuate acoustic energy above a selected frequency. An example of such construction can be found in U.S. Pat. No. 2,757,358 granted to J. O. Ely on July 31, 1956. In this Ely patent, the support member consists of a plurality of spaced weights connected to one another by a plurality of tubes. While this structure will operate to attenuate high-frequency acoustic energy, there will be a direct path for low-frequency acoustic energy through the tubes such that this low-frequency energy will arrive at the receiver before the acoustic energy which travels through the formations.
It is recognized that a support member of an acoustic logging apparatus should be able to both delay and attenuate acoustic energy travelling longitudinally of the support member. At the same time, the support member should have a significant amount of rigidity and strength because of the harsh downhole conditions to which it will be subjected. Thus in designing such a support member, there are a number of conflicting considerations which must be taken into account. To enhance the delay characteristic of the support member, it would be desirable to provide as many slots as possible therein. However, the more slots that are placed in the support member, the weaker will be the structural characteristics thereof. If a plurality of weights are placed in the skin of the tubular member thus improving its filtering characteristics, its structural characteristics will also be worsened. Thus to provide the required structural characteristics for the support member, it has in the past been necessary to restrict the efficiency of the support member in delaying and attenuating acoustic energy travelling longitudinally of the member.
It is therefore an object of the present invention to provide new and improved acoustic logging apparatus wherein the support member has high-strength qualities and improved attenuation and delay characteristics for minimizing transmission of acoustic energy along the support member.
The above discussion has been concerned with the type of acoustic well logging apparatus wherein the acoustic transducers are mounted on the longitudinally extending support member. In addition to this form of acoustic logging apparatus, it is also possible to mount some or all of the acoustic transducers on a pad member which is pressed against the wall of the borehole for investigating the adjoining formations. Such apparatus can be found in U.S. Pat. No. 3,406,776 granted to L. Henry on Oct. 22, 1968.
In constructing such a pad member, it is sometimes important that it maintain good strength characteristics and have the ability to attenuate or delay acoustic energy travelling along the length of the pad member. One might suggest that the attenuating and delaying structure of the patents discussed above could be applied to such pad structures. However, the small size of the pad member makes this very difficult.
It is therefore another object of the present invention to provide a new and improved pad structure for acoustic well logging apparatus which has high strength characteristics as well as the ability to attenuate acoustic energy.